Get rid of the unused native code StringAddStub.

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index aed2a28a9b13a24e0c715471a80e8ecaef4fad1b..00954b8e133c9e8d47b13f0e252d1ed85c84702a 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -4444,18 +4444,11 @@ void LCodeGen::DoTrapAllocationMemento(LTrapAllocationMemento* instr) {
 
 void LCodeGen::DoStringAdd(LStringAdd* instr) {
   ASSERT(ToRegister(instr->context()).is(cp));
-  if (FLAG_new_string_add) {
-    ASSERT(ToRegister(instr->left()).is(a1));
-    ASSERT(ToRegister(instr->right()).is(a0));
-    NewStringAddStub stub(instr->hydrogen()->flags(),
-                          isolate()->heap()->GetPretenureMode());
-    CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
-  } else {
-    __ push(ToRegister(instr->left()));
-    __ push(ToRegister(instr->right()));
-    StringAddStub stub(instr->hydrogen()->flags());
-    CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
-  }
+  ASSERT(ToRegister(instr->left()).is(a1));
+  ASSERT(ToRegister(instr->right()).is(a0));
+  StringAddStub stub(instr->hydrogen()->flags(),
+                     isolate()->heap()->GetPretenureMode());
+  CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
 }